1. Technical Field of the Invention
This invention relates to a semiconductor laser unit capable of emitting a plurality of laser beams and an optical head device equipped therewith, and more particularly to a semiconductor laser unit and optical head device improved in the focusing characteristic on the plurality of laser beams.
2. Description of the Related Art
An optical head device has been widely used for recording to or reproducing from an optical disk such as a DVD (Digital Versatile Disc), a CD (Compact Disc) or the like. An optical head device 100 shown in FIG. 10, which is a conventional example, is equipped with a conventional semiconductor laser unit 101 capable of emitting a plurality of laser beams, so-called a multi-beam semiconductor laser unit. A half mirror 102 is arranged on a light-emission side of the semiconductor laser unit 101. A collimator lens 103 and an objective lens 104 are arranged in the order on a light-reflection side of the half mirror 102. At or around a focal spot of the objective lens 104, an optical disk 105 or 106 is arranged to record thereto or reproduce therefrom. Herein, the optical disk 105 is, for example, a DVD which is small in protection layer thickness, while the optical disk 106 is, for example, a CD which is greater in protection layer thickness than a DVD. A photo-detector 107 is arranged on a light-transmission side of the half mirror 102 as viewed from the collimator lens 103.
The semiconductor laser unit 101 used on the optical head device 100 includes a unit shown in JP-A-2001-298238, hereinafter referred to as a first background art. The semiconductor laser unit 101 shown in FIG. 11 as the first background art includes a case 108, a heat block 109 attached on the case 108, a sub-mount 110 arranged on the heat block 109, and a multi-beam semiconductor laser unit 111 arranged on the sub-mount 110. The multi-beam semiconductor laser unit 111 has light-emission points 112 and 113 spaced by approximately 100 microns. The first light-emission point 112 which emits a shorter laser beam in wavelength than the second light-emission point 113 is in a center of an outer shape of the case 108. The semiconductor laser unit 101 is arranged on the optical head device 100 such that the laser beam emitted from the first light-emission point 112 is coincident in its axis with an optical axis 114 of the collimator lens 103 and the objective lens 104 shown in FIG. 10. For example, the light-emission points 112 and 113 emit laser beams at a wavelength of 650 nm for recording to or reproducing from DVD and at a wavelength of 780 nm for recording to or reproducing from CD, respectively.
The laser beam, emitted from the first light-emission point 112 of the semiconductor laser unit 101 shown in FIG. 11, is reflected upon the half mirror 102 shown in FIG. 10, followed by entering the collimator lens 103 where it is converted into a collimated laser beam. Thereafter, this enters the objective lens 104 and is focused onto the optical disk 105. The laser beam reflected upon the optical disk 105 travels through a reverse optical path to pass through the half mirror 102, followed by being irradiated onto the photo-detector 107. In the photo-detector 107, detected are the reproduced signals from the optical disk 105 and the signals required for focusing and tracking. Similarly, the laser beam, emitted from the second light-emission point 113 of the semiconductor laser unit 101, is focused onto the other optical disk 106. In the photo-detector 107, detected are the reproduced signals from the optical disk 106 and the signals required for focusing and tracking.
As for the first light-emission point 112, high-performance focusing characteristic is available without influence of an aberration of the collimator lens 103 and objective lens 104, because the axis of the laser beam emitted from the first light-emission point 112 exists on an optical axis 114. On the other hand, as for the second light-emission point 113, because the axis of the laser beam emitted therefrom dose not exist on the optical axis 114, incidence is oblique on the collimator lens 103 and objective lens 104, thus being unavoidably influenced by an aberration. However, a CD, irradiated by a laser beam longer in wavelength emitted from the second light-emission point 113, has a greater margin than a DVD in respect of wavelength and lens focusing performance, and therefore, there is no significant trouble in practical operation.
JP-A-2002-25103, a second background art, discloses a similar optical head device to the first background art. This shows that, in mounting a semiconductor laser unit having two light-emission points for emitting laser beams of different wavelengths on an optical head device 100, a light-emission point emitting a laser beam of a shorter wavelength (first light-emission point 112) is coincident on an optical axis 114 or arranged so that a distance between the light-emission point emitting a laser beam of a shorter wavelength (first light-emission point 112) and the optical axis 114 may be shorter than a distance between the light-emission point emitting a laser beam of a longer wavelength (second light-emission point 113) and the optical axis 114. In this case, similarly to the first background art, the light-emission point emitting a laser beam of a shorter wavelength (first light-emission point 112) is arranged to a position close to the optical axis 114 with priority over the light-emission point emitting a laser beam of a longer wavelength (second light-emission point 113), because it is readily influenced by an aberration of the collimator lens 103 and the objective lens 104.
In the meanwhile, in the optical head device 100, an astigmatism generally exists on the multi-beam semiconductor laser unit 111 besides the aberration resulting from a positional relationship between the lens system (herein, collimator lens 103 and objective lens 104) and the light-emission point. The astigmatism is preferably suppressed to a possible small extent because it also has an effect upon a focusing characteristic.
JP-A-2000-251314, a third background art, discloses a conventional optical head device shown in FIG. 12, in which references 100 to 107 and reference 114 are the same as those shown in FIG. 10. An astigmatism compensating plate 115 is arranged on an optical path between a semiconductor laser unit 101 and a half mirror 102. The astigmatism compensating plate 115 is in the form of a parallel plate formed of a transparent optical member, such as glass or resin. As shown in FIG. 13, the plane formed by the axes 116 and 117 of the laser beams emitted from two light-emission points 112 and 113, respectively, is on the same plane (X-Z plane in FIG. 13) as the normal line of an incidence surface of the astigmatism compensating plate 115. Also, the astigmatism compensating plate 115 is arranged inclined relative to the plane formed by the axes 116 and 117 of the laser beams.
It is apparent that, in case a divergently emitting laser beam transmits through the parallel-plated astigmatism compensating plate 115 arranged with inclination relative to the axes 116, 117, an astigmatism occurs in the laser beam. The amount of astigmatism occurrence is influenced to a refractive index of an optical member structuring the astigmatism compensating plate 115, parallel plate thickness and an inclination angle in arrangement the astigmatism compensating plate 115. In the case that an astigmatism already exists in the laser beam entering the astigmatism compensating plate 115, added thereto is an astigmatism caused due to the transmission through the astigmatism compensating plate 115. Therefore, a refractive index, thickness and inclination angle of the astigmatism compensating plate 115 are set in a manner to give an astigmatism reverse in direction to and same in magnitude as the amount of the astigmatism existing in the laser beam entering the astigmatism compensating plate 115, so that the astigmatism can be suppressed in the laser beam after transmitted through the astigmatism compensating plate 115. As noted above, because the astigmatism generally exists on the multi-beam semiconductor laser unit 111 itself, it is very effective to adopt such an astigmatism compensating plate 115.
Furthermore, the astigmatism compensating plate 115 applied to the multi-beam semiconductor laser unit 111 also has an effect shown in JP-A-2001-237501, a fourth background art. Namely, for the laser beams different in wavelength emitted from light-emission points 112 and 113, their axes 116 and 117, are refracted at both the incident and exit surfaces of the astigmatism compensating plate 115. Generally, the refractive index of an optical member depends on a wavelength, i.e. increases with decrease in wavelength. Accordingly, the laser beam 116 shorter in wavelength undergoes greater refractive action to have a greater parallel shift amount in the laser beam axis 116 before entering and after exiting from the astigmatism compensating plate 115 than that in the axis 117 of the laser beam longer in wavelength. As shown in FIG. 13, in the case that the astigmatism compensating plate 115 is arranged with inclination in a manner nearing the light-emission point 113 for emitting a laser beam of longer wavelength, the distance B between the axes 116 and 117 of the two laser beams after exiting from the astigmatism compensating plate 115 is smaller than the distance A between the axes 116 and 117 of the two laser beams before entering the astigmatism compensating plate 115. Conversely, in the case that the astigmatism compensating plate 115 is arranged with inclination in a manner nearing the light-emission point 112 for emitting a laser beam of shorter wavelength, the distance B between the axes 116 and 117 of the two laser beams after exiting from the astigmatism compensating plate 115 is greater than the distance A between the axes 116 and 117 of the two laser beams before entering the astigmatism compensating plate 115. In this manner, the distance between the axes 116 and 117 of the two laser beams can be varied to entering and exiting from the astigmatism compensating plate 115 by changing the inclination angle of the astigmatism compensating plate 115. It is accordingly possible to freely adjust a positional relationship between the two light-emission points 112, 113 and the optical axis 114.
However, the conventional semiconductor laser unit 101 and the optical head device 100 equipped therewith have involved the following problems.
In the first and second background arts, the light-emission point 112 for emitting a laser beam shorter in wavelength is arranged on or close to the optical axis 114 in order to preferentially secure the focusing performance, for example, for the DVD strict in aberration condition as compared to the CD. However, there is a problem of sacrificing the focusing performance on a laser beam longer in wavelength emitted from the light-emission point 113.
Meanwhile, the third background art is structured such that the astigmatism existing on the two light-emission points 112, 113 of the multi-beam semiconductor laser unit 111 is compensated by the use of the astigmatism compensating plate 115. Accordingly, in the case that the two light-emission points 112, 113 emit laser beams of the same wavelength and have the same astigmatism each other, compensation can be easily made on the two light-emission points 112, 113. However, where the two light-emission points 112, 113 emit laser beams different in wavelength, they do not necessarily have the same amount of astigmatism, because the materials structuring the light-emission points 112, 113 of the multi-beam semiconductor laser unit 111 are different. Accordingly, there is difficulty in simultaneously compensating the astigmatisms on the two light-emission points 112, 113 where there is difference in the astigmatisms. Thus, further required is means for securing a focusing performance on a light-emission point more insufficient of compensation among the two light-emission points.
Meanwhile, in the fourth background art, the astigmatism compensating plate 115 is arbitrarily inclined to adjust to a desired value the distance B between the axes 116 and 117 of the two laser beams after transmitting through the astigmatism compensating plate 115. However, there occurs variation in the amount of astigmatism of the laser beam after transmitting through the astigmatism compensating plate 115 due to the inclination angle of the astigmatism compensating plate 115. Thus, there is a problem that much time is required in measuring a distance B or in an adjusting process for the astigmatism compensating plate 115.
Furthermore, the first to fourth background arts, in any, merely explain to obtain the optimal focusing characteristic on the optical axis 114 of the collimator lens 103 and the objective lens 104 of the optical head device 100, but are silent in the relationship with a focusing characteristic including an astigmatism occurring on the above lens system. Accordingly, in the viewpoint of strict optical design, there is a problem that structuring is not made to simultaneously optimize the focusing characteristic on the two laser beams.